1. Field of the Invention
The present invention relates to a point-to-point optical fiber link for transmitting digital signals from and towards a user apparatus, within a network for distributing signals to a plurality of users.
The present invention also relates to a network for distributing signals to a plurality of users and a method for connecting, in a signal distribution network, an optical cable to an opto-electronic conversion device.
2. Description of the Related Art
Currently, in the field of telecommunications, optical technology is mainly used for long-distance transmission of optical signals wherein the known properties of wide band offered by optical fibers are exploited. For the transmission of signals (such as digital television, telephone and/or data signals) from and towards users and the transmission of digital data between electrical apparatuses (such as Personal Computers) of a Local Area Network (LAN), the technology mainly used is, however, that in which electrical cables, such as coaxial cables or cables consisting of copper pairs, are used.
However, electrical cables have a relatively narrow band and are becoming a bottleneck with respect to the band of the signals to be transmitted. Moreover, they have problems of electromagnetic interference, of impedance matching and are difficult to insert into the suitable raceways of a building since they are rigid. Moreover, since they are bulky, they substantially limit the number of cables that can be inserted in a raceway. Moreover, for electrical safety requirements, they require the provision of separate raceways from those used for the distribution of electrical energy.
Research interest is therefore steering towards the possibility of using optics not only in the long-distance signal transmission, but also in networks for distributing signals from a common branching point to a plurality of user apparatuses. Fiber optic cables, indeed, are suitable for being inserted in the suitable raceways of a building since they are not too bulky, are flexible, light, free from electromagnetic interference and have low bending loss. Moreover, they are suitable for being inserted in the same raceways used for the distribution of electrical energy. Moreover, optical fibers potentially have a very wide band, low attenuation values and they are transparent to the bit rate, format and transmission code.
However, for the connection to electrical apparatuses, fiber optic cables require the use of opto-electronic devices to convert the optical signals transported thereby into corresponding electrical signals and vice-versa.
Throughout the present description and claims the expression “opto-electronic conversion” is used to indicate an optical-electric and/or electric-optical conversion.
The conversion of an optical signal into a corresponding electrical signal is conventionally carried out through a photodetector whereas the conversion of an electrical signal into a corresponding optical signal is conventionally carried out by modulating the intensity of a light emitted by a light source according to the information transported by the electrical signal.
The connection of an optical fiber to a light source and/or to a photodetector is conventionally carried out through an optical connector. Typically, an optical connector is a device comprising two parts that can be repeatedly connected and disconnected to each other and that must be attached one to an end of the optical fiber and the other one to a pig-tail of the light source or of the photoreceiver.
For example, the installation of an optical cable comprising an optical fiber, suitable for implementing a point-to-point link within a building, between a user apparatus and a distribution unit (situated, for example, in an office or apartment and, respectively, in the cellar or loft) requires the following steps to be performed: passage of the optical cable along a suitable raceway of the building; cutting of the optical cable according to the necessary length; application of the connectors to the two ends of the optical fiber at the user apparatus and at the distribution unit; possible application of the connectors to a light source and to a photodetector (in the case in which the light source and the photodetector are not already equipped with connectors); and, finally, connection through the connectors of one end of the fiber to the light source and of the other end of the fiber to the photodetector, at the distribution unit and at the user apparatus side.
An alternative known technique for connecting an optical fiber to an opto-electronic device is a fused junction between an end of the optical fiber and a pigtail of the opto-electronic device.
However, both optical connectors and fused junction require the use of fiber stripping (elimination of the protective outer coating), cleaving and polishing operations, which are very delicate to carry out on site, require high precision (in the range of micrometers), highly specialized personnel and tools and thus involve high installation times and costs. Moreover, these types of operations must be carried out at the premises of the user, very often in inconvenient and narrow spaces (for example, under a table or desk).
Therefore, although fiber optic cables have numerous advantages, their use in networks for distributing signals to a plurality of users has up to now been highly limited both due to the high installation costs and because the optical connection may be unreliable if the aforementioned operations are not carried out correctly.
In order to solve this problem, WO 01/50644 describes a signal distribution network comprising fiber optic cables that are electrically terminated, i.e. having at least one end permanently connected to an opto-electronic terminal portion.
However, this solution has the disadvantage of requiring lager spaces for the passage of the opto-electronic termination along the raceways of a building with respect to those required by the cable not yet opto-electronically terminated. Moreover, also in the case in which the cable is electrically terminated at just one end, such a solution still requires the use of highly specialized personnel and tools to connect the other end of the cable to an opto-electronic termination.
Therefore, the Applicant faced the problem of reducing the installation costs of a fiber optic network for distributing signals to a plurality of users.
The Applicant has found that such a problem can be solved by using a technique that allows an optical signal to be extracted and injected from/into a fiber through suitable bending of the optical fiber. Indeed, the Applicant has observed that with this technique the optical fibers can be used, for the terminations, complete with their protective coating, provided that the same is sufficiently transparent; that the terminal face of the optical fiber carries out no optical function and that the aforementioned critical stripping, cleaving and polishing operations of the fiber, required by the use of optical connectors or fused junctions, are eliminated. Moreover, by using a suitable locking mechanism to keep the fiber bent and in optical alignment position with respect to the opto-electronic device, the connection operations of the optical fiber to the opto-electronic device can be carried out by any technician, even if not specialized in optical cable terminations, or, indeed, even by the final user.
Extraction and injection techniques from/into fiber of an optical signal through bending of the optical fiber are known in the art.
For example, U.S. Pat. No. 4,950,046, GB 2 236 405, U.S. Pat. Nos. 4,696,534 and 4,696,535 describe devices for extracting and/or injecting light in fiber through bending of the same to be used to locally inject out and/or tap optical signals in order to carry out an active alignment of two waveguides or optical fibers or to detect whether an optical signal is present in an optical fiber, for example during maintenance.
U.S. Pat. No. 4,950,046 describes a device in which the fiber is bent with a banding radius R equal to 2.8 mm; GB 2 236 405 describes a bending radius R comprised between 1 and 3 mm; U.S. Pat. No. 4,696,534 describes a bending radius R comprised between 3 and 10 mm and U.S. Pat. No. 4,696,535 describes a bending radius R comprised between 3 and 10 mm.
Moreover, Loke et al. (“Simulation and measurement of radiation loss at multimode fiber macrobends”, Journal of Lightwave Technology, Vol. 8, No. 8, August 1990, pages 1250-1256), show the results of simulations and experiments carried out to determine the macro-bending losses of multi-mode fibers. They show that the macro-bending losses of an optical fiber are inversely proportional to the bending radius R.
The Applicant has observed that both the injection efficiency and the extraction efficiency of an optical signal into/from a bent optical fiber increase as the bending radius decreases. However, too small bending radii (for example between 1 and 2.5 mm) subject the optical fiber to high stresses that can lead in time to the breaking thereof.
Therefore, although the solutions described by the aforementioned patent documents can be used for the applications described therein, in which the optical fiber must be kept in bent position only for the time required by the specific application (i.e. just for the duration of the active alignment operation or of the maintenance operation to be carried out), they are not suitable for being used for long-lasting applications. In particular, they are not adapted to be used in an optical signal distribution network that uses a fiber optic point-to-point link, to connect the ends of the optical fiber to the opto-electronic conversion apparatuses where the optical fiber must be kept in bent position for the lifetime of the point-to-point link.
U.S. Pat. No. 4,768,854 describes the use of the fiber bending technique to extract and inject an optical signal in a signal distribution network comprising non-destructive read taps arranged in series which create extremely small attenuations of an optical signal being tapped. In this document it is stated that a permanent bend in a multi-mode optical fiber that has a minimum bend radius of 3.5 mm over a 45° sector angle creates less than a 10% chance of fracturing the fiber section in 20 years, and a 3.8 mm minimum bend radius over a 45° sector angle creates less than 1.5*10−2% chance of fracturing the fiber section in 20 years. Moreover, it is stated that bend radii equal to or greater than 4.2 and 4.5 mm result in much lower breaking probability.
The Applicant observes that this document teaches to use bending radii of more than 3.5 mm and preferably more than 4.2 mm to obtain breaking probability values of less than 10% in 20 years to the detriment, however, of the extraction and injection efficiency of the optical signal from/into fiber.
Therefore, the Applicant faced the technical problem of reducing the installation costs of a network for distributing signals to a plurality of user electrical apparatuses in an effective and reliable manner.